AMK said:

I’m going with a TN system with electrical separation as the output of the UPS in battery mode is floating with respect to earth, but the exposed conductive parts of the equipment are still earthed.

If the exposed-conductive-parts are still Earthed, that would be an IT system, not electrical separation. [And in this case, you can't rely on the distributor's means of earthing as discussed above.]

In electrical separation, there is no connection between PE [exposed-conductive-parts] and the general mass of Earth.

Udtwmc said:

I should add that in this particular installation the equipment earths are bonded to a PE terminal within the UPS enclosure so don’t solely rely on the UPS load side earth I.e. the ups could be unplugged and PE connection remain intact). 

If "floating" (connection with Earth lost but exposed-conductive-parts are connected to a live conductor) that system is not defined in IEC 60364, HD 60364 or BS 7671 at the present time, and potentially not [always fully] compliant. This is because there is a specific fault condition that can lead to exposed-conductive-parts and connected metalwork becoming immediately hazardous-live with respect to Earth. I believe there has been at least one case of death (and prosecution) resulting from similar circumstances.

However, that doesn't mean it's always unsafe .,.. importantly , it can be unsafe under certain specific conditions. For this reason, protection by RCD that disconnects all live conductors, as close as possible to the source (in this case the UPS inverter) is imperative ... but that doesn't necessarily mean 30 mA.

There are odd occasions, such as the supply of a single EV from a single transformer that are cited in BS 7671 as compliant. In other cases, e.g. BS 7909, where the installation is under constant supervision, or specific cases in BS 7430, it may also be acceptable.

Work is ongoing to research this type of system, but in the mean-time, using it outside the use-cases in Section 722 of BS 7671, or BS 7909, or BS 7430, might lead to issues.

It is imperative for BS 7671 compliance in this situation (Regulation 551.4.3.2.1) that, unless the supply is from private transformers (where the installation has its own earthing system) there must be an additional consumer's earth electrode connected to MET.

That system is not defined in IEC 60364, HD 60364 or BS 7671 at the present time. So is the system a hybrid TN/electrical separation? What would you call the current setup ? So, When the UPS is running normally or as a backup, the installation uses the TN system and the protection is (ADS). When the power fails the UPS/battery becomes the new supply source, and the installation switches to the IT system ? So options are use monitoring devices to detect the first fault to earth but this option requires that the UPS output neutral is separated from earth but all the class I devices are connected to earth. Or use electrical separation to ensure that each class I device is supplied by its own individual circuit from the UPS output and that there is no common connection between them. The UPS output neutral is connected to earth but each class I device is not and each device is individually protected?

AMK said:

That system is not defined in IEC 60364, HD 60364 or BS 7671 at the present time. So is the system a hybrid TN/electrical separation? What would you call the current setup ? 

So, provided it's connected to an Earth electrode belonging to the consumer, it's TN-S.

If it's connected to the MET, which is only earthed by an incoming DNO supply, not usually permitted under BS 7671 (and BS 7671 is specified by ESQCR in Great Britain and the NI version in Northern Ireland) if you have switched alternative supplies like a UPS. See Regulation 551.4.3.2.1.

If it's not connected to a means of earthing, again doesn't comply with BS 7671.

Point of discussion: some people think that either 551.4.3.2.1 and/or Regulation 21 of BS 7671, doesn't apply to a UPS because a UPS is not a "switched alternative supply" ... BUT ... that would only be true for a UPS with no bypass switching arrangement.

so treat it like an isolated generator configuration, common on small single phase generators. No connection to the mass of earth or between neutral and protective conductor, an I-S system. with the protective conductor connection to earth then would be classified as an IT system. On the I-S system with a number of class 1 equipment, each item should have its own RCD.

I've had a reply from Schneider APC regarding my model of UPS (SRT2200RMXLI)

[APC] "Since both poles will be disconnected from the supply thus creating an IT system. The supply has then no reference to the earth so the first fault is not a safety hazard."

Agree disagree??

based on their statement if i'm to follow BS 7671: 411.6 [IT system when in battery mode] I have a question on what is meant by 411.6.1 NOTE 1 "It is strongly recommended that IT systems with distributed Neutrals should not be employed"  ...

So I'm not sure I understand the meaning of this when a neutral has to be employed for 230VAC equipment?  or is it the that my two live conductors from the UPS are now deemed L-L not L-N

And regarding the point on regulation 551.4.3.2.1  I read this as only being relevant when the generating set (my UPS) provides a supply for 'distribution to the public' ... 

firstly, the UPS is only feeding several fixed items of equipment (no socket outlets or other DB boards) so no one can easily utilise the power from the UPS for their own means -  is this still distribution? 

and secondly, do the limited number of skilled authorised personel in the plant room qualify as 'public' ?

Well they have confirmed it does what you think, and floats the output. Given it is feeding a conventionally wired building, it is not really supplying a pure IT system, but for sure you can short the live to chassis of some kit, or to terra-firma, and nothing trips until the power is restored to mains so as they say,  first fault is a no trip.

It is indeed not really what '7671 recommends for a UPS.

However, it is not that dangerous.,,

Mike.

Disconnection  in the event of a first line fault to earth is not essential. In the event of two faults precaution must be taken. All exposed conductive parts are required to be earthed and The sum of the earth electrode and protective conductor connecting exposed conductive parts multiplied by the fault current between line and part needs to be less than or equal to 50 V. The conductors are deemed L1 and L2 on the output side of  a isolating transformer.
Distributed  neutrals can cause harmonics,power quality issues, problems with electromagnetic compatibility. 

Udtwmc said:

And regarding the point on regulation 551.4.3.2.1  I read this as only being relevant when the generating set (my UPS) provides a supply for 'distribution to the public' ... 

Nope, it's when the input to the UPS is connected to a public supply, and effectively the bypass is used to supply the downstream ("UPS supplied") equipment from the public supply. In that case, the bypass switching is the equipment that switches between UPS output and public supply - the UPS output is the "switched alternative".